The property possessed by certain metal chelates of catalyzing the conversion of mercaptans to disulfides when they are placed in an alkaline medium and in the presence of oxygen is known to have long had wide application. (French Pat. Nos. 996,851 and 1,230,502.)
It is also known that of these chelates, the metal phthalocyanines are frequently used in a fixed bed on certain activated carbons (French Pat. No. 1,301,844) for the sweetening of heavy gasolines and of kerosenes. In fact, certain petroleum fractions contain straight-chain or branched-chain mercaptans, or mercaptans of an aromatic nature, and/or contain tertiary and polyfunctional mercaptans of high molecular weight, which are only sparingly soluble in the alkaline solution required for the sweetening reaction. Also, petroleum fractions containing mercaptans to be oxidized are generally sweetened in the following manner: The hydrocarbon to be sweetened is mixed with the oxidizing agent (usually air), then treated in a fixed-bed reactor containing an adsorbent support of high specific surface which has previously been impregnated and over which the alkaline reactant, generally an aqueous solution of soda, is passed, either continuously at a very low rate or discontinuously, to maintain the alkalinity required for the oxidation reaction.
Among the supports generally regarded as potentially usable in the preparation of a composite catalyst capable of forming a fixed bed, a large number of solid materials is mentioned in the literature, including activated carbons obtained by the pyrolysis of wood, peat, lignite, bone or various other carbonaceous materials; natural clays and silicates, such as diatomaceous earth, fuller's earth, kieselguhr, attapulgite, feldspar, montmorillonite, halloysite and kaolin; and natural or synthetic refractory oxide minerals such as alumina, silica, the oxides of zirconium, thorium and boron, or mixtures thereof.
To be suitable for industrial use, however, a good support for sweetening catalysis must possess a number of intrinsic properties, which limit the choice of potential supports. In particular, the adsorbent support should
be insoluble in soda and inert to the petroleum fraction to be sweetened; PA1 have enough active sites to bind the metal chelate effectively to its surface; PA1 have a sufficiently high pore volume to permit the bulkiest mercaptans to be contacted with the metal chelate with which it is impregnated; and PA1 have sufficiently high crushing strength to be able to withstand variations in the operating conditions imposed by the stresses of industrial operation. PA1 carbon precursors such as wool waste or waste of any other hydrocarbonaceous compound of the same nature, PA1 refractory silicates or metallic oxides which are chemically inert and after treatment are capable of forming the framework of a solid support having good crushing strength, and PA1 soluble mineral salts, such as the salts of potassium or sodium in particular,
Also, in practice only certain types of activated carbons have gained acceptance as catalyst supports, particularly since they may accelerate the oxidation of mercaptans under certain conditions even in the absence of any active component. (U.S. Pat. Nos. 2,872,412 and 2,908,637.) In particular, the activated carbons used up to now have a specific gravity which generally ranges from 0.2 to 0.5, a specific surface between 300 and 2500 m.sup.2 /g, a micropore volume between 0.3 and 0.6 cm.sup.3 /g, and an ash content ranging from 1 to 20 percent.
However, the industrial use of composite sweetening catalysts whose support is an activated carbon has shown that the choice of such a support entails a number of drawbacks so far as industrial operation is concerned. In fact, it has been found that sweetening reactors must be taken off-stream frequently to wash the composite (in principle with hot water) and to remove so far as possible the organic sodium salts and the gums formed on its surface by peroxidation of the hydrocarbons present in the feedstocks to be sweetened. Since the regeneration of the composite catalyst so effected is far from complete, the catalytic potential of the metal chelates deposited on activated carbons is not fully utilized. The service life of the composite in industrial operation with certain petroleum feedstock then is sharply reduced, which in view of the high cost of the activated carbon used as support can detract from the economy of the process.